Somebody stole my mood ring.

Somebody stole my mood ring.
I'm not sure how I feel about that.

In order to understand this joke, you have to understand what a mood ring is. Mood rings are devices that became popular in the 1970's that change color based on their temperature. Supposedly mood rings were able to notify the user of their state of emotions. The jokester in this case is making a statement that he or she is unable to tell what their emotions are without the aid of a mood ring.

This is patiently absurd, because mood rings are a gimmick, and do not actually tell you anything about your emotional state.

Other than that, it is sort of hard to categorize this joke.